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Late Cenozoic crustal extension and magmatism, southern Death Valley region, California

By
J.P. Calzia
J.P. Calzia
U.S. Geological Survey, 345 Middlefield Road, Menlo Park, California 94025 USA
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O.T. Rämö
O.T. Rämö
Department of Geology, University of Helsinki, P.O. Box 11, FIN-00014, Finland
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Published:
January 01, 2000

Abstract

The late Cenozoic geologic history of the southern Death Valley region is characterized by coeval crustal extension and magmatism. Crustal extension is accommodated by numerous listric and planar normal faults as well as right-and left-lateral strike slip faults. The normal faults dip 30°-50°W near the surface and flatten and merge at depth into a detachment zone at or near the contact between Proterozoic cratonic rocks and Proterozoic and Paleozoic miogeoclinal rocks; the strike-slip faults act as tear faults between crustal blocks that have extended at different times and at different rates. Crustal extension began 13.4-13.1 Ma and migrated northwestward with time; undeformed basalt flows and lacustrine deposits suggest that extension stopped in this region (but continued north of the Death Valley graben) between 5 and 7 Ma. Estimates of crustal extension in this region vary from 30-50 percent to more than 100 percent.

Magmatic rocks syntectonic with crustal extension in the southern Death Valley region include 12.4-6.4 Ma granitic rocks as well as bimodal 14.0-4.0 Ma volcanic rocks. Geochemical and isotopic evidence suggest that the granitic rocks get younger and less alkalic from south to north; the volcanic rocks become more mafic with less evidence of crustal interaction as they get younger.

The close spatial and temporal relation between crustal extension and magmatism suggest a genetic and probably a dynamic relation between these geologic processes. We propose a tectonic-magmatic model that requires heat be transported into the crust by mantle-derived mafic magmas. These magmas pond at lithologic or rheoslogic boundaries, begin to crystallize, and partially melt the surrounding crustal rocks. With time, the thermally weakened crust is extended (given a regional extensional stress field) concurrent with granitic magmatism and bi-modal volcanism.

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GSA Field Guide

Great Basin and Sierra Nevada

David R. Lageson
David R. Lageson
Department of Earth Sciences Montana State University Bozeman, MT 59717 USA
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Stephen G. Peters
Stephen G. Peters
Reno Field Office Mackay School of Mines, MS-176 University of Nevada Reno, Nevada 89557-0047 USA
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Mary M. Lahren
Mary M. Lahren
Department of Geological Sciences MS-l72 University of Nevada Reno, Nevada 89557 USA 2000
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Geological Society of America
Volume
2
ISBN electronic:
9780813756028
Publication date:
January 01, 2000

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